The chemical protection of metallic surfaces such as copper and its alloys from oxidation is well known in the art. The use of various triazoles and imidazoles as anti-oxidants has been known for some time. In fact Benzotriazole is known and has provided excellent anti-oxidation properties for copper and its alloys. These and other anti-oxidants have proved successful as long as the protected surface is not heated or subjected to prolonged exposure.
With the rise of surface mount technology (SMT) in printed circuits boards, the anti-oxidants of the prior art have proven deficient at the high temperatures involved in the multiple soldering operations frequently necessary to produce printed circuit boards with this technology. Antioxidants (or prefluxes) which also protect solderability at elevated temperatures have therefore been sought to fill this need. Therefore, the need to provide a preflux which is easily applied and stable at high temperatures, and thus protects the solderability of copper circuitry is clear. Several advances have been made in this area utilizing alkyl-benzimidazoles. U.S. Pat. No. 4,395,294 reveals the use of 5-methyl benzimidazole. Although this is an improvement on the use of benzotriazole, 5-methyl benzimidazole forms only a very thin protective layer on the copper surfaces and thus its anti-corrosion and solderability properties at high temperatures are insufficient to meet today's needs.
The leading technology in this area today is taught in recently issued U.S. Pat. No. 5,173,130, the disclosure of which is incorporated herein by reference. This patent teaches the use of a benzimidazole compound having an alkyl group of at least 3 carbon atoms at the 2-position in conjunction with an organic acid. The materials taught in this, the Kinoshita, patent are greatly improved over the prior art. Kinoshita provides for improved thermal resistance through the provision of a benzimidazole with a long alkyl chain in the 2-position. The long alkyl chain which Kinoshita stresses as necessary for good thermal resistance causes other problems, however. The long alkyl chain is also very hydrophobic and therefore causes the compounds taught by Kinoshita to have very limited aqueous solubility. This low solubility then causes precipitation of the Kinoshita compounds in aqueous solution ms the solution is used. This obviously causes problems in production, consistency and life.
Thus the need still exists for a pre-flux, anti-oxidant, which possesses excellent thermal resistance and which has good water solubility.